Ceramic foams are known for various applications, in particular more recently as supports for catalytically active materials fulfilling several requirements simultaneously, as described in "Preparation and properties of ceramic foam catalyst supports" by M V Twigg and J T Richardson published in the "Scientific Bases for the preparation of heterogeneous catalysts" 6th International Symposium Sep. 5-8 1994 Louvalne La Neuve, Belgium. Open pore ceramic foams are materials with high temperature resistance, low bulk density and tortuous flow patterns by virtue of connecting adjacent pores or "cells" providing non-linear channels. Ceramic foams enable the passage of gases at high space velocities and acceptable pressure drop but they do not offer the surface area available with conventional catalyst forms such as extrudates. Commercially available foams may have a BET surface area (as defined in "Adsorption surface area and porosity" S J Gregg & K S W Sing, Academic Press London 1982) of typically less than 1 m2/g, in particular of about 0.2 or 0.3 m2/g after high temperature calcination for a prolonged period, which is too low to be useful in the majority of catalytic applications. A high surface area is generally accepted to be advantageous in providing a high surface area of catalytically active material per unit bed volume, in particular for operation of high pressure drop conversion processes in which a high catalyst bed volume can lead to the need for excessive space velocities. Twigg and Richardson report an alumina washcoating technique for the stabilization of a 30 pore per inch alpha-alumina/mullite ceramic foam at high temperature, whereby a four-fold increase in surface area of the ceramic foam was achieved after calcining at 1000.degree. C. for 4 hours.
U.S. Pat. Nos. 4,810,685 and 4,863,712, corresponding to EP 0 260 826, disclose ceramic foam supports suited for use in steam reforming of methane, comprising a network of irregular passages extending therethrough, comprising supported catalytically active material and an inorganic oxide stabilizer to prevent sintering of the active material. The stabilizer and the active material are introduced by impregnation of the foam by means of immersion of the foam in an aqueous solution of a salt of the stabilizer and the active component, draining to remove excess solution and firing at 450.degree. C. This process is repeated to build up sufficient impregnant layer on the foam. However the foams described are to be used at relatively low temperatures, of the order of 760.degree. C., and may not give the desired stabilization at higher temperatures. This process is suited for the stabilization of the active component in the existing (low) surface area foam, but does not address enhancing the surface area of the existing low surface area foam. The application of this process for enhanced surface area of the low surface area foam is laborious and time consuming.
FR 2 590 887 (no U.S. equivalent found) discloses zirconium oxides having stable surface area at elevated temperatures, the oxide comprising as additive an oxide of silica, the rare earths, yttria, ceria and/or aluminum. The additive may be introduced by various means including co-precipitation, mixing of salt with sol hydrate and impregnation of the zirconium oxide with a salt precursor of the additive. Impregnation is preferably performed "dry" whereby the total volume of the impregnating solution is approximately equal to the total pore volume of the (oxide) support. It is taught by means of example to impregnate extruded support granules with the aqueous impregnating solution, to dry at 160.degree. C. for 16 hours and calcine at 400.degree. C. The additive is nevertheless present in low amounts of 1 to 10wt % based on the weight of the total composition. The support may be in the form of granules or pellets. The BET surface area is increased from 20 m2/g without additive to 40-50 m2/g with additive at 900.degree. C. This document discloses the impregnation of mesoporous structures, wherein the additive is adsorbed and crystallized onto the support. There is no reference to supports in the form of foam structures, which comprise pores of several orders of magnitude greater than the mesoporous, and for which a different mechanism of supporting of the additive is involved.
The provision of ceramic foams having significant increase in BET surface area remains a problem. There is a need for a ceramic foam structure and preparation thereof which is suited to the extreme conditions encountered in some processes, and moreover which exhibits improved stability when compared with known foam supports.